1. Field of the Invention
The present invention relates to an optical fiber and a process of producing the same, more particularly relates to an optical fiber having a core cross-section shape suited to optical connection with a light source such as a semiconductor laser emitting an elliptical beam and a process of producing the same.
2. Description of the Related Art
In the field of optical measurement using optical fibers or the field of optical communications, various means have been devised to ensure that the beam of light from the light source efficiently strikes the core of the optical fiber.
In the fields of optical communications etc., a semiconductor laser (or a laser diode (LD)) is used as the light source in many cases. Usually, a beam of light with an elliptical cross-section shape is emitted from the end of the LD. In the past, the core of a single mode (SM) optical fiber has been formed with a circular cross-section. To get the incident light beam from the LD directing into an optical fiber with such a core, use has been made of an optical system comprised of special lenses in order to make the elliptical beam of light emitted from the LD a circular beam of light before emitting it into the core of the optical fiber.
Using an optical system to change the beam shape of light in this way, however, necessitates the provision of a complicated optical system, so the price becomes higher and more space is taken up as well. Further, optical loss occurs in the optical system.
Accordingly, attempts have been made to use a mode field converting optical fiber or a spot size converting optical fiber. A mode field converting optical fiber or spot size converting optical fiber has a core of an elliptical cross-section at the light receiving side at its one end and a core of a circular cross-section at the light emitting side at its other end. The elliptical beam of light emitted from the LD is directly fired into the elliptical core at the one end (light receiving side) of the optical fiber and a circular beam of light is emitted from the circular core at the other end (light emitting end) of the optical fiber.
In such a mode field converting optical fiber or spot size converting optical fiber, the elliptical beam of light is changed to a circular beam of light along the longitudinal direction of the optical fiber.
A process for production of a spot size converting optical fiber has been proposed for example, in Japanese Unexamined Patent Publication (Kokai) No. 3-64707.
The process for production of the spot size converting optical fiber will be explained next referring to FIG. 1A to FIG. 1C.
As illustrated in FIG. 1A, an elliptical core-type optical fiber 40 comprised of a core 41 on the outer circumference of which is formed a cladding 42 is prepared. The dimensions of the core 41 at one end are a long axis diameter of 5 .mu.m and a short axis diameter of 2 .mu.m giving a substantially elliptical shape close to that of a rectangle. If heat is applied at the side of this elliptical core type optical fiber, the core 41 having the ellipse cross-section gradually becomes rounded and finally becomes a core 41A having a circle cross-section, between the heated portion and the portion to which the heat-transfer is not substantially subjected or effective, as shown in FIG. 1B. As a result, at the one end, as shown in FIG. 1A, the core is remained at the elliptical core 41, and at the other end, as shown in FIG. 1C, the core becomes the circular core 41A. The diameter of this circular core is 3.6 .mu.m. By applying the heat, the dopant, that is, the germanium (Ge), contained in the core 41 disperses and the spot size becomes 11.2 .mu.m. That is, the process proposed in Japanese Unexamined Patent Publication (Kokai) No. 3-64707 changes the core to a circular shape by heat dispersion of the elliptical core.
This process, however, suffers from the difficulty in changing what is actually close to a rectangular elliptical core to a true circular core. As a result, the problem is encountered of a large transmission loss occurring in the light propagated inside the optical fiber.
Further, even if the elliptical core could be changed to a truly circular core, this method suffers from the disadvantage of a reduction in the difference in the indexes of refraction between the core and the cladding due to the dispersion of the dopant and therefore a reduction in the efficiency of connection.
An example of the conventional process of production of a mode field converting optical fiber will be explained next. An optical fiber having an elliptical core and an optical fiber having a circular core are connected at their ends. If necessary, the connection is heated to cause the dopant at the core portions to disperse and to form a portion along the longitudinal direction which gradually changes in shape from an elliptical core to a circular core. Accordingly, the result is an optical fiber with an elliptical core at one end, a circular core at the other end, and a core portion which gradually changes from an ellipse to a circle along the longitudinal direction of the optical fiber.
This process too, however, encounters the problem of disconnection between the elliptical portion and circular portion of the core and the occurrence of large transmission loss in the light propagated through the inside of the optical fiber.
Further, even if the elliptical core can be changed to a truly circular core, there is the problem of a reduction in the difference in the indexes of refraction between the core and the cladding due to the dispersion of the dopant and therefore a reduction in the efficiency of connection.
As explained above, both the process for production of the mode field converting optical fiber and the process for production of the spot size converting optical fiber suffer from the problem of a large transmission loss in the optical fiber produced. Further, they have the problem of a reduction in the difference in the indexes of refraction between the cores and claddings.